1. Field of the Invention
The present disclosure relates to infrared detection or imaging, and especially to micro-bolometric detection. More specifically, the present invention relates to techniques enabling to adjust the spectral characteristics of packages containing the sensitive detection elements.
2. Description of Related Art
Detectors well known in the art intended for infrared imaging or thermography comprise a sensitive retina, placed in the focal plane of optics and integrated in a usually tight package, which protects the retina, and maintains it in a very low pressure envi-ronment usually necessary for it to operate properly. The retina is formed of a two-dimensional assembly of sensitive unit elements or “pixels”, formed at the surface of an electronic chip called “read circuit” (ROIC) comprising circuitry for addressing and forming the signal of each pixel. These signals altogether thus form the electric image in relation with a thermal scene, observed by means of an adapted optical system which focuses the thermal image onto the sensitive retina.
The package wall, placed on the optical path between the optics and the retina, usually called “window”, is formed of a planar substrate essentially transparent to the radiation of interest, that is, between 8 μm and 14 μm for infrared detection adapted to the most current case of observation of scenes in the vicinity of 300 K.
To optimize, on the one hand, the transmission in this band, and on the other hand to limit the transmission outside of the band, and more specifically for wavelengths shorter than 8 μm, optical filters are usually applied to the two surfaces of the window.
As low a transmission as possible between 2 μm and 8 μm is indeed desired, to avoid for the detector to respond in the spectrum where the sensitivity depends on the observation distance between the emitting object and the focal plane, and accordingly to avoid for the detected infrared intensity to be dependent from the limited atmospheric transparency on this portion of the spectrum. Further, limiting the transmission between 2 μm and 8 μm enables not to expose the sensitive elements of the retina to energetic radiations from very hot sources which may appear in the observed field, and cause temporary or even permanent damage to the sensitive elements.
Usually, and practically, the optical fibers formed on the window surfaces are formed of interference stacks comprised of many thin layers of predetermined thickness, each essentially transparent between 2 and 20 μm, with an alternation of layers of low refraction index, usually made of zinc sulfide, and of layers of high refraction index, usually made of germanium. It is thus possible and known to control and to optimize to some extent the transmission spectrum of the air-substrate interface of the window on a first surface, and to optimize the substrate—air (or vacuum) transmission on the opposite surface.
The quality of a filter is further determined by the characteristics of the defects, especially splinters, scratches, inclusions, and other structure defects contained by said filter. Indeed, such defects and imperfections are capable of disturbing the images formed by the detector, and this all the more as the filters are close to the focal plane, and thus to the sensitive detection elements. It should be noted that as such, defects of the filter formed on the surface of the window closest to the focal plane more strongly disturb the images formed by the detector than defects of the filter formed on the other surface of the window.
Now, modern detectors precisely tend to have smaller and smaller dimensions, in particular in terms of thickness, that is, along the optical axis, especially to make the best of optical systems having a very short focal distance. As detectors miniaturize, filters having defects of smaller and smaller dimensions and having a defect surface density which is also smaller and smaller, should thus be provided.
However, such increasingly harsh specifications become an economical issue since the cost of these optical filters increases rapidly due to the specific method precautions to be taken and/or to the associated efficiency losses. The obtained performance or the costs induced by this approach are not adapted yet to the need relative to the field of current infrared detection components.
Decreasing the thickness of optical filters is an approach currently explored to improve their quality. Indeed, the number and the size of structure defects increase along with the filter thickness. Thus, a decrease of their thickness results in a substantial decrease of the size and of the number of defects, as well as of the cost of the multilayer optical processing. However, the decrease of the total thickness typically goes along with a decrease of the number of layers used, which necessarily results in a slackening of spectral transmission specifications. This approach thus results in accepting a degradation in terms of filter performance and of susceptibility of the detector to very hot sources, in return for a lower cost and defects decreased in size and number, that is, a better defectivity.
Another approach of the state of the art to improve the defectivity comprises implementing a microstructured two-dimensional network by direct etching of the window surface(s). This technique is for example described in documents WO 02/054499 and US 2009/080075. This technique is however very seldom implemented, in particular due to the cost of the associated necessary techniques and instruments. Further, the simplicity of such structures does not enable to define transmission spectrums such as desired for infrared bolometric detection. Indeed, the equivalent of one or, at best, two interference layers is obtained in this manner, and only an antireflection function is thus obtained, which drastically limits the general optimization potential of the obtained filter.
It thus remains necessary to form a usual structure by multilayer processing on the other surface of the window to obtain a low transmission between 2 and 8 μm, with the previously-mentioned disadvantages.